As video transmission systems have matured, digital video is more readily available via a variety of different communications systems and networks. Specifically, digital video, such as television programs, can be transmitted as multicast digital bit streams of video signals to users over networks. Changing between channels of the multicast digital bit streams can often result in significant delays and interruption in user presentation experience.
Multicast digital bit streams typically include digital video frames. A predetermined number of frames is conventionally referred to as a Group of Pictures (GOP). The GOP lengths are typically 15 or 30 frames. With more advanced video formats, such as Advanced Video Coding (AVC) and/or Windows Media 9 (WM9), the GOP length can be substantially longer in order to reduce the bit rate.
In order to reduce costs and simplify the amount of effort associated with video transmission, different video compression/de-compression techniques have been developed and established. Some of the better known and more widely adopted video compression/de-compression standards include Motion Picture Experts Group 2 (MPEG-2) data streams and Motion Picture Experts Group 4 (MPEG-4) data streams. Hence, television programs are often transmitted over the network as Motion Picture Experts Group 2 (MPEG-2) data streams or Motion Picture Experts Group 4 (MPEG-4) data streams. Conventionally, for purposes of video compression/decompression, a video stream is processed one frame at a time.
Compressed video transmission streams typically include a variety of different compression frame types. With MPEG-2 and MPEG-4, the bit streams generally include three different types of frames including Intra-frames, Predictive frames, and Bidirectional interpolated frames. In a typical decoding process, Intra-frames (I-frames) can be decoded independently without the need of referencing another frame. Thus, GOPs typically start with an I-frame. Predictive frames (P-frames) can be decoded by referencing a previous I-frame or P-frame. Bidirectional interpolated frames (B-frames) can be predicted from a previous and a following P-frame or I-frame. For a given video stream, all three ways of coding are attempted and the best and most efficient combination is utilized. For example, a common MPEG-2 video stream can be 15 frames long and have the sequence IBBPBBPBBPBBPBB.
Typically, a video stream, such as a MPEG-2 data stream, is transmitted from a multicast source to a router and/or switch via a network, e.g., an Internet Protocol (IP) distribution network. And upon receipt of the video stream, the router then transmits the video stream to a user device, such as a set-top box.
However, unfortunately, live multicast video streams transmitted over a network, e.g., an IP distribution network, are frequently affected by start-up delays when a user changes channel. Start-up delays can be caused by video stream characteristics, buffer fill-up time, and/or the time it takes to acquire video stream specific information. Video stream specific information is a periodic update and interleaves with other data. For example, conventional channel change operations wait to acquire video stream specific information and the beginning of the next GOP before presenting content, which often results in relatively long delays.
In particular, a single factor that can cause a considerable set-top box startup delay is the length of the Group of Picture involved. Traditionally the set-top box startup time is directly related to the lengths of a Group of Picture (GOP) from the new channel. Thus, typically the longer the GOP length, the longer the startup delay time is for a set-top box. As a result, in general, the start-up delay associated with GOP length prevents a user from being able to transition from one channel to another without appreciable delay. Consequently, the multimedia experience is affected and made less enjoyable.